1. Field of the Invention
This invention relates to electrochemical devices including, for example, lithium secondary cells and more particularly, to electrochemical devices which make use of lithium nitride-metal compounds as at least one electrode thereof.
2. Description of the Prior Art
Investigations have been hitherto made on transition metal oxides such as, for example, Li.sub.x CoO.sub.2, Li.sub.x NiO.sub.2, Li.sub.x MnO.sub.2 and the like, for use as an active material for the positive electrode of lithium secondary cells. Likewise, substances having various types of layered structures, such as graphite, have been studied for use as an active substance for the negative electrode.
Where these substances are, respectively, employed as the active substance for the electrodes, their crystal structures greatly influence the electrochemical oxidation and reduction cycles of the cell. For instance, the crystal structure of the transition metal oxide such as LiNiO.sub.2 is a kind of layered structure wherein infinite chains of edge- or apex-sharing tetrahedra or octahedra, in which transition metal element ions are bonded with oxygen ions through six or four coordinations, extend two-dimensionally. In the layered structure, lithium elements are intercalated. When a substance having such a layered structure as mentioned above is subjected to electrochemical polarization, lithium ions are intercalated in or de-intercalated from the layered structure and act as an active substance for the electrode.
In case where substances having the layered structure as mentioned above are used as an active substance for electrode, the electrochemical reaction which takes place will be a so-called topochemical reaction wherein ions are merely intercalated in or de-intercalated from the crystal layers of the layered structure. As a consequence, any appreciable change in the crystal structure does not take place, so that high reversibility is ensured with respect to the repetition of the electrochemical redox reaction cycles.
However, the use of the transition metal oxides as the active substance has the following problems to solve.
The crystals of transition metal oxides and graphite have such a two-dimensional structure as set out hereinabove. The materials used as the active substance have a limited, finite dimension. The two-dimensional structure breaks off on the surface of the material. With transition metal oxides such as LiNiO.sub.2, the oxygen ions become "terminated", for example, in the form of --OH, --OLi or other end groups.
If these materials are used as the active substance, the electrochemical reactions include not only the intercalation and de-intercalation reaction of Li.sup.+ ions relative to the crystal layers, but also additional reactions ascribed to the H.sup.+ and/or Li.sup.+ ions of the end groups. This surface state may change during the working cycles of the electrochemical cell or during storage of the cell. This will result in the formation of end groups which are liable to impede the intercalation and/or de-intercalation reaction of the Li.sup.+ ions, thus bringing about an increase in impedance of the interface between the electrodes and the electrolyte. Hence, the electrochemical reaction does not proceed smoothly. With the cell, the degradation of performances such as a lowering of charge and discharge capacities will result.
The transition metal oxides have been described as having a layered structure. Graphite also has a layered structure wherein hexagons of C.sub.6 extend two-dimensionally. More particularly, graphite has such a surface state that has end groups, such as quinone groups, ketone groups and the like groups, bonded thereto. Graphite materials have been heretofore investigated for use as an active material for the negative electrode of lithium secondary cells. Like the transition metal oxides, changes take place in the end groups after repetition of the charge and discharge cycles, resulting in the degradation of performance of the cell. In addition, the type of end group depends greatly on the history of the graphite. This will bring about various side reactions which will occur along with the intended charge and discharge reactions. This presents the following problems.
In general, the end groups in or on the graphite surfaces are, in most cases, liable to suffer electrochemical reduction. At the first charge reaction, gases will be generated owing to the electrochemical reduction of the end groups and an electrolyte. Eventually, the inner pressure of the cell undesirably increases at the initial stage of the charge cycle.
As will be appreciated from the problems involved in the transition metal oxides and graphite, where materials having a layered structure are used as active substances to constitute electrochemical cells, there arises the problem that the performance of the cell is degraded owing to the reactions in which end groups take part.